Pigmentary TiO2 is commercially produced through the sulfate or the chloride process. The chloride process is also used to produce TiCl4 for titanium metal production. In the chloride process, titanoferrous ore is carbochlorinated to produce TiCl4 and a range of other metal chlorides from the ore impurities. The crude TiCl4 produced in the carbochlorination is processed with a series of physical separation steps to produce a usable TiCl4 product. One contaminating element found in titanoferrous ore is arsenic. The chlorination of the arsenic species present in the ore produces AsCl3. AsCl3 has a boiling point very similar to that of TiCl4, making removal more problematic.
Different ores can contain significantly different levels of arsenic ranging from non-detectible to greater than 100 ppm. Standard purification methods for the chloride process involve first removing solids chlorides and then removing vanadium in a separate step. AsCl3 is a liquid, so it is not removed by the solids removal steps. Known vanadium removal steps such as organic treating agents, like plant and animal oils, soaps, fats and waxes, do not react with AsCl3. Another known commercial process is using elemental copper to remove vanadium from crude TiCl4. Copper also shows no reactivity to AsCl3. As a result, all of the AsCl3 that forms from chlorination is present in the pure TiCl4 sent to oxidation and can end up in the TiO2 product. High levels of arsenic are undesirable in TiO2 pigment. Pigmentary TiO2 used in FDA products such as cosmetics require <1 ppm arsenic by the FDA method. Low levels are also desired in other pigmentary application such as some plastics and coatings products. Arsenic levels in TiCl4 used to produce titanium metal must also be kept low to avoid deformations in the final metal pieces. Typical levels for TiCl4 for titanium metal are <10 ppm arsenic.
Since AsCl3 passes through all the known vanadium removal processes, such as organic treatment or copper metal, all the AsCl3 will end up in the purified TiCl4. If high concentrations of arsenic were present in the ore, elevated levels of AsCl3 will also be present. Two technologies are known to remove AsCl3 from pure TiCl4. If a partial reduction of the concentration from, for example, 100 ppm to 10 ppm is all that is required, distillation can be used with effective production of the desired product, but a significant yield loss of TiCl4 is also required. Lower concentrations can also be achieved at greater penalties for energy consumption and equipment sizing. AsCl3 has little commercial value. Arsenic is currently only used in a few specific applications, and each of these requires a high purity level, such as gallium arsenide production. As a result, using distillation of produce a highly concentrated AsCl3 product would reduce the yield loss of TiCl4 but would not yield a useful product. The AsCl3/TiCl4 stream would need disposal in a proper manner. Since the boiling points of AsCl3 and TiCl4 are so close, only 6° C. apart, a large amount of energy would be required to produce this waste stream.
Another potential method for removing AsCl3 from purified TiCl4 is to use carbon adsorption. This method does not work on crude TiCl4. Carbon adsorption can remove the AsCl3 to very low levels that would be suitable for all applications including cosmetics. However, the carbon adsorption is not selective for only AsCl3. Many other species are present in the pure TiCl4 such as the sulfur gases produced from the carbochlorination, like SO2, COS, and CS2. These species will adsorb competitively on to the carbon, limiting the capacity. As a result, this method is not commercially viable for large scale production such as pigmentary TiO2 for large markets like plastics and coatings.
Thus, the problem to be solved is removal of AsCl3 from TiCl4 produced via the chloride process in an economical, efficient, and safe manner.